Patent Application
20160141975
The present disclosure relates to a capacitor drop power supply circuits and power supply methods.
A capacitor drop power supply provides a simple and low cost way for converting an AC voltage such as a mains voltage to a DC supply voltage, which may be used for driving a load. Instead of providing a transformer to step down the voltage, a capacitor (known as a drop capacitor) is coupled in series with the AC supply and acts to step down the voltage. Power supplies of this type are used in various contexts, for example as auxiliary supplies for moter drivers and in electrical appliances.
An illustrative schematic of a typical capacitor drop power supply is shown in
Despite the low cost and simplicity of a drop capacitor power supply, the practical implementation of such a circuit is limited by a number of problems.
Firstly, the circuit must be designed to deal with a range of voltages around a nominal output voltage value that is to be output by the circuit. The drop capacitor must have sufficient capacitance to deliver enough power at a minimum voltage in the range. Therefore, at the nominal voltage the drop capacitor delivers more current than is needed and so excess energy is dissipated in the zener diode.
Also, power dissipation does not depend on the load. If the load does not consume energy, the energy will be dissipated in the zener diode. This restricts use of the capacitor drop power supply in applications with low standby power consumption requirements.
The drop capacitor has lower impedance for higher harmonics of the AC line frequency. If a capacitor drop power supply is coupled with a supply that has significant high frequency harmonic content, the power dissipation in the zener diode and other components could exceed predicted values resulting in circuit overheating and failure.
It is therefore desirable to provide a non-isolated power supply topology which will outperform competitive solutions in cost and performance.
According to a first aspect of the disclosure there is provided a capacitor drop power supply circuit for coupling with an input AC supply and providing a DC output voltage, said circuit comprising a drop capacitor, and a rectifier circuit comprising a switch that is selectively operable to regulate the DC output voltage.
Optionally, the capacitor drop power supply circuit comprises:
i) a rectifier circuit with an input and an output;
ii) a drop capacitor provided between a first AC supply terminal and the input of the rectifier circuit; and
iii) a filter capacitor provided between the output of the rectifier circuit and a second AC supply terminal;
iv) wherein the rectifier circuit comprises:
v) a diode coupled between the drop capacitor and the rectifier circuit output;
vi) a switch connected between the rectifier circuit input and the second AC supply terminal; and
vii) a controller which can selectively operate the switch to regulate an output voltage of the rectifier circuit.
When one component is provided between other components, this can be via a direct coupling or alternatively the coupling may be indirect, in other words the provision of additional interposing components is not precluded.
Optionally, switching a rectifier circuit to regulate the DC output voltage is achieved using a semiconductor switching element.
Optionally, the rectifier circuit provides a half wave rectified output.
Optionally, the rectifier circuit provides a full wave rectified output.
Optionally, the controller provides trailing edge control.
Optionally, the controller provides leading edge control.
According to a second aspect of the disclosure there is provided a power supply method comprising converting an AC supply to a DC output by coupling the AC supply with a drop capacitor; and selectively switching a rectifier circuit to regulate the DC output voltage.
Optionally, the rectifier circuit comprises an input and an output, a diode coupled between the drop capacitor and the rectifier circuit output, a switch connected between the rectifier circuit input and the second AC supply terminal, and a controller; and the method comprises:
i) providing a drop capacitor between a first AC supply terminal and the input of the rectifier circuit;
ii) providing a filter capacitor provided between the output of the rectifier circuit and a second AC supply terminal; and wherein
iii) the controller selectively operates the switch to regulate an output voltage of the rectifier circuit.
Optionally, the switch comprises a semiconductor switching element.
Optionally, the rectifier circuit provides a half wave rectified output.
Optionally, the rectifier circuit provides a full wave rectified output.
Optionally, the controller provides trailing edge control.
Optionally, the controller provides leading edge control.
The disclosure will be described below, by way of example only, with reference to the accompanying drawings, in which:
In an existing capacitor drop power supply, all the energy stored in the drop capacitor is either consumed by the load or dissipated in the zener diode. Referring to the circuit of
The present disclosure provides a capacitor drop power supply circuit where excess charge is damped into a low impedance switch. The low impedance switch is provided in place of a zener diode and so the dissipation of extra energy is avoided.
An embodiment of the disclosure is schematically illustrated in
During a positive half cycle of the AC supply 300, AC current passes through the drop capacitor 310 and rectifier diodes 306, 308 and to the parallel combination of the output and filter capacitor 314. The filter capacitor 114 is charged by the current flow and when the charge reaches a certain threshold the switch 312 is closed. Excess current is then damped in the switch 312 while the filter capacitor 314 remains charged and the output voltage across the terminals 302, 304 remains constant. During this time the drop capacitor 310 is charged and its voltage increases. The switch 312 remains closed throughout the course of the time when excess energy is being provided by the drop capacitor 310 and filter capacitor 314, so that the excess energy does not get dissipated. Then, when the capacitors 310, 314 are no longer supplying excess energy, the switch opens again. The switch may open again during the positive half cycle or during the negative half cycle.
Because the excess charge is not dissipated, it then becomes available for when a load is applied thus increasing the efficiency of the power supply as compared with a topology in which energy is dissipated in a zener diode.
A controller is provided to operate the switch. The present disclosure is not limited to any one type of controller, but as an example a switch controller comprises a comparator that provides trailing edge current control to the rectifier diode 306. An embodiment of this is illustrated in
In the embodiment of
This type of the controller provides trailing edge current control to the rectifier diode 306.
To illustrate the advantages of the circuit of
With the conventional capacitor drop power supply design of
During normal operation this capacitor will deliver 100 mA current. The zener diode should be able to dissipate energy at a full power value: 12V*0.1 A=1.2 W. The efficiency at full load is 83% and in standby mode is 8.3%.
A simulation was carried out which took into account factors including power dissipation of the rectifier diodes 106, 108 and it was found that a full load efficiency of 74% was achieved.
For the improved design, according to the embodiment of
Use of trailing edge control has a disadvantage. The theoretical efficiency is limited by energy dissipated in the switch during turn on. These losses are indicated as spikes on the capacitor current waveform.
By using more complicated leading edge control it is possible to implement zero voltage turn on soft switching. The theoretical efficiency of this scheme is 100% as there is no discharge of the capacitor and no energy losses associated with it.
After AC input voltage passes its positive peak, the voltage on the input of the synchronization circuit 726 (diode 708) starts to reduce. When this voltage becomes negative, diode 708 becomes forward biased and can start to conduct the current. At the same time the synchronization circuit 726 resets the voltage on the ramp capacitor 730 (Cramp). This marks the beginning of the switching cycle. A current source 732 discharges the ramp capacitor 730, creating a negative slope.
Because the voltage of the ramp 728 is applied to a positive input of the PWM comparator 724, its output will switch into a high state and the switch 712 will be turned on. Current from the drop capacitor 710 will go through the low impedance of the switch 712 without significant power dissipation.
After the AC input voltage passes its negative peak, current through the drop capacitor 710 will reverse direction, but still goes into the switch 712.
When the ramp voltage crosses the output voltage of the error amplifier 722, the PWM comparator 724 will change state and will turn switch 712 off. Current though the drop capacitor 710 will not be shorted by S1 and will flow to the load through diode 706 until the AC voltage reaches its positive peak.
If the output voltage of the error amplifier 722 is lower, the time during which current flows to the load is less, so output voltage will reduce. If the error amplifier 722 output is higher, the output voltage will increase. This function combined with the inverting function of the error amplifier 722 will create the negative feedback. To ensure stable feedback a compensator circuit should be employed, ideally a type II proportional-integral (PI) compensator.
The present disclosure therefore provides various advantages compared with existing capacitor drop power supplies. The various embodiments of the disclosure provide the simplicity and low cost of a capacitor drop power supply, but with an efficiency that is equivalent or superior to that of a switching mode power supply. Furthermore, because extra energy is not dissipated in the power supply of the present disclosure, lower capacitor impedance will not cause extra power loss meaning that the present disclosure allows for the use of low cost capacitive drop techniques with mains supplies that have a high harmonic content.
Therefore the present disclosure provides power supplies that can close the market niche between capacitor drop and switched mode power supplies.
Various modifications and improvements can be made to the above without departing from the scope of the disclosure. While aspects of the invention have been described with reference to exemplary embodiments, it is to be clearly understood by those skilled in the art that the invention is not limited thereto.
